1. Field of the Invention
This invention relates to the field of charged particle optics, and in particular to charged particle optics components for semiconductor wafer and mask defect inspection systems.
2. Description of the Related Art
Charged particle beam systems employed for imaging purposes typically generate a primary beam, of electrons or ions, which is focused onto the surface of a substrate by probe-forming optics. The imaging process generally involves the collection of secondary electrons which are emitted from the substrate surface as a result of the interaction of the primary charged particle beam with the substrate surface. In imaging systems, the energy of the primary charged particle beam striking the substrate surface is generally at least several hundred eV, while the secondary electrons leaving the substrate surface have energies predominantly below 10 eV. In order to form an image of the substrate, it is necessary to separate the secondary electrons from the primary charged particles and collect these secondary electrons with some type of detector.
In many charged particle beam systems, the secondary electron detector is positioned within the probe-forming optics, and a crossed magnetic-electric field filter (commonly called a Wien filter) is used to deflect the secondary electrons off-axis into a detector, while simultaneously allowing the primary beam to pass through the Wien filter undeflected. The main limitation to this approach is that the Wien filter introduces some aberrations into the primary beam, since only one energy in the primary beam is undeflected and the primary beam (with nominal energy e V0) will inherently have a spread in energies (±e ΔV) due to the electron source—this is particularly a problem in low voltage columns, where the fractional energy spread (ΔV/V0) is larger and often unacceptable. Another disadvantage of this approach is cost, since the Wien filter requires both current supplies for the magnetic coils and voltage supplies for the electrostatic electrodes.
In other charged particle beam systems, the secondary electron detector is positioned below the probe-forming optics, and off to one side of the optical axis of the probe-forming optics. In these systems, it is necessary that the electric fields from the secondary electron detector do not substantially affect the primary beam. This requirement typically limits the secondary electron collection efficiency.
In charged particle beam systems where the primary beam energy is low, such as electron beam inspection systems, achieving high secondary electron collection efficiencies is particularly demanding. Clearly, there is a need for improved detector optics which provides high secondary electron collection efficiency combined with minimal distortion of the primary charged particle beam.